The invention relates to an antenna for transmitting microwaves, having a circular waveguide short-circuited at the end on one side, in which circular waveguide a transmitting element for feeding in microwaves is arranged, a funnel-shaped horn adjoining an open end of the circular waveguide, the radius of which horn increases in the direction facing away from the circular waveguide, and an insert made of a dielectric, which insert has a first cylindrical section, which fills the circular waveguide, and which insert has a conical section, whose tip points in the direction facing away from the circular waveguide.
Such antennas, which are usually referred to as horn antennas, are used e.g. in filling level measuring technology for determining a filling level of a charge material in a container. In this case, microwaves are transmitted by means of an antenna to the surface of a charge material and the echo waves reflected at the surface are received. It goes without saying that an antenna used for transmission can likewise be used for reception. An echo function which represents the echo amplitudes as a function of the distance is formed, and from this the probable useful echo and the propagation time thereof are determined. The distance between the charge material surface and the antenna is determined from the propagation time.
In order to determine the filling level, it is possible to use all the known methods which enable comparatively short distances to be measured by means of reflected microwaves. The best known examples are pulsed radar and frequency modulated continuous wave radar (FMCW radar).
In the case of pulsed radar, short microwave transmission pulses, referred to below as wave packets, are transmitted periodically, are reflected from the charge material surface and, after a distance-dependent propagation time, are received again. The received signal amplitude as a function of time constitutes the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a specific distance from the antenna.
In the case of the FMCW method, a continuous microwave is transmitted, said microwave being periodically linearly frequency-modulated, for example according to a sawtooth function. The frequency of the received echo signal therefore has a frequency difference with respect to the instantaneous frequency of the transmission signal at the instant of reception, said frequency difference depending on the propagation time of the echo signal. The frequency difference between the transmission signal and the reception signal, which can be obtained by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance between the reflecting surface and the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum which is obtained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum therefore constitutes the echo function in this case.
In the case of conventional horn antennas, reflections frequently occur within the antennas. Reflections lead to alteration of the microwaves to be transmitted, e.g. to a short microwave pulse that is to be transmitted being artificially lengthened. Furthermore, these undesired reflections bring about an increase in the signal background and, consequently, a deterioration in the signal/noise ratio.
Microwaves to be transmitted may also be altered by virtue of different microwave modes and/or components of a microwave signal or microwave packet having a different frequency propagating at different speeds in the coupling-in region and/or in the horn. This can also result e.g. in the widening of a short microwave pulse.
Together with the horn, the conical section of the insert effects a comparatively continuous transition of the impedances from the circular waveguide through to free space. This matching can be considerably improved by the choice of a suitable material for the insert, but it is not completely loss-free even then.
DE-U 298 12 024 describes an antenna for transmitting microwaves, having
a circular waveguide short-circuited at the end on one side,
in which circular waveguide a transmitting element for feeding in microwaves is arranged,
a funnel-shaped horn adjoining an open end of the circular waveguide,
the radius of which horn increases in the direction facing away from the circular waveguide, and
an insert made of a dielectric,
which insert has a first cylindrical section,
which fills the circular waveguide, and
which insert has a conical section,
whose tip points in the direction facing away from the circular waveguide
In order to improve the signal quality of the antenna, in the case of this antenna the horn was arranged at a distance from the conical section.
An object of the invention is to specify an antenna which radiates microwaves to be transmitted as far as possible uncorrupted into free space and which operates in a low-loss manner.
To that end, the invention consists in an antenna for transmitting microwaves, having
a circular waveguide short-circuited at the end on one side,
in which circular waveguide a transmitting element for feeding in microwaves is arranged,
a funnel-shaped horn adjoining an open end of the circular waveguide,
the radius of which horn increases in the direction facing away from the circular waveguide, and
an insert made of a dielectric,
which insert has a first cylindrical section,
which fills the circular waveguide, and
which insert has a conical section,
whose tip points in the direction facing away from the circular waveguide and
whose base area has a diameter which is greater than a diameter of the cylindrical section.
In accordance with one refinement of the invention, the horn is at a distance from the conical section in the radial direction.
In accordance with one development of the invention, a second cylindrical section is arranged between the first cylindrical section and the conical section, and the diameter of said second cylindrical section is equal to the diameter of the base area of the conical section.
In accordance with one refinement of the invention, the conical section has in the axial direction a length which is approximately equal to the wavelength of microwaves to be transmitted in free space.
The invention and its advantages will now be explained in more detail with reference to the figures of the drawing, which illustrate two exemplary embodiments; identical parts are provided with identical reference symbols in the figures.